Electromagnetic-acoustic transducers (EMATs) have been used in non-destructive testing, including in the borehole, using well-understood physical phenomena. When a wire is placed near the surface of an electrically conducting object and is driven by a current at a suitable ultrasonic frequency, eddy currents are induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents experience Lorentz forces. These forces cause an acoustic excitation in the object. In a reciprocal use, an electric signal will be generated in the wire as a result of acoustic excitation in a metal placed close to a permanent magnet. Attenuation and/or reflection of the acoustic waves bear information on the defects and surroundings of the object. An EMAT may be designed to produce a single waveform, such as shear horizontal waves (SH) or Lamb waves.
Electromagnetic-acoustic transducers (EMATs) have more recently been used downhole as components of downhole tools. The transducer periodically emits an acoustic energy pulse on command from a controller circuit in the tool. After emission of the acoustic energy pulse, the transducer can be connected to a receiving circuit, generally located in the tool, for measuring a returning echo of the previously emitted acoustic pulse which is reflected off the borehole wall. By processing the reflected signal, it is possible to infer something about the acoustic impedance characterizing the near-borehole environment. Specifically, changes in acoustic impedance are diagnostic of the geometry of the borehole.
As one example, it is known to conduct acoustic inspection of a casing cemented in a borehole to determine specific properties related to the casing and surrounding materials. For example, the bond between the cement and the casing may be evaluated, or the strength of the cement behind the casing or the casing thickness may be estimated using measurements of reflected acoustic waves, which may be generally referred to as casing cement bond logging